{ // 通过 “.” 和 “[]” 两种方式引用对象的属性，注意两者的区别

    let o = {x: 1, y: {z: 3}}; // An example object
    let a = [o, 4, [5, 6]]; // An example array that contains the object

    o.x          // => 1: property x of expression o
    o.y.z        // => 3: property z of expression o.y
    o["x"]       // => 1: property x of object o
    a[1]         // => 4: element at index 1 of expression a
    a[2]["1"]    // => 6: element at index 1 of expression a[2]
    a[0].x       // => 1: property x of expression a[0]
}
